Folk Forecasting

Want to know what the weather will be like this year? Maybe you should ask an Andean potato farmer to go out at night and take a peek at the Pleiades.

Potatoes, the staple crop in the Andes, need a lot of moisture. The best planting time is at the start of the rainy season in October, but in drought years the rains arrive weeks late. Farmers traditionally determine when the rains will come by noting the brightness of the Pleiades star cluster months earlier. The folk ritual really works, say anthropologist Benjamin Orlove of the University of California at Davis and atmospheric scientists John Chiang and Mark Cane of the Lamont-Doherty Earth Observatory in New York.

The Pleiades contain 11 visible stars of varying brightness. In June in the Andes, the cluster appears low over the eastern horizon just before dawn. If the skies are clear, the cluster will appear brighter and contain more visible stars, which farmers take as a sign to plant on time. High cirrus clouds make the Pleiades dimmer, which tells the farmers that rain is likely to arrive late. "It's quite a dramatic thing. They go out at about 4 a.m. during the coldest time of their year and climb mountains to get unobstructed views," Orlove says. When he and his colleagues consulted satellite imagery and climate data, they found the farmers' technique is very sensitive to high, obscuring clouds--the atmospheric herald of drought and El Nino.